Generator sizing Updated June 2026
What Size Generator to Run a Window AC Unit?
By Yann Buf, HVAC tech 5 min read Standards-reviewed
A 5,000 BTU window AC needs a 1,200W generator, a 10,000 BTU unit needs 2,400W, and a 15,000 BTU unit needs 3,500W. Always factor in the starting surge (approximately 2x running watts).
AC Starting Surge: The Most Common Generator Sizing Mistake
Air conditioners have compressor motors that require a significant surge of power to start — typically 2-3 times their running wattage. This is the most common reason generators fail to power AC units. A 10,000 BTU window AC might draw 1,200W while running, but it can spike to 2,400W or more when the compressor kicks on.
The key is to size your generator for the starting watts, not the running watts. If you want to run a window AC plus a refrigerator and some lights, you need a generator that can handle all the running watts plus the highest single starting surge — which is almost always the AC compressor.
For most homeowners who want to run a 10,000 BTU window AC plus basic essentials (fridge, lights, fan), a 3,500W generator provides comfortable headroom.
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Generator Sizing Guide by AC Unit Size
- 5,000 BTU window AC (small bedroom): 500W running, ~1,200W starting → 1,500W generator
- 8,000 BTU window AC (large bedroom/small living room): 800W running, ~1,800W starting → 2,500W generator
- 10,000 BTU window AC (living room): 1,200W running, ~2,400W starting → 3,500W generator
- 12,000-15,000 BTU window AC (large room/garage): 1,500W running, ~3,500W starting → 5,000W+ generator
Frequently asked questions
Can my generator run both a window AC and a refrigerator?
Yes, as long as the generator can handle both running watts plus the highest starting surge. A 3,500W generator can run a 10,000 BTU window AC (2,400W starting surge) and a refrigerator (600W starting surge) simultaneously, since the surges won't happen at the same time.
Do inverter generators handle AC starting better?
Yes. Inverter generators provide cleaner power and can handle short-duration surges better than conventional generators. For sensitive electronics and AC units, an inverter generator is a better choice.
Should I use a soft start kit for my window AC?
A soft start kit ($100-200) reduces the starting surge of an AC compressor by 30-50%. This can allow a smaller generator to run a larger AC unit. If you're trying to power a 12,000 BTU unit with a 2,500W generator, a soft start kit may make it possible.